This invention relates to a method and apparatus for generating a wipe solid.
Referring to FIG. 1 of the accompanying drawings, a wipe generator is used in conjunction with a video mixer 2, which receives two synchronized input video signals VideoA and VideoB, to provide a control signal such that the output video signal VideoC of mixer 2 contains a transition from VideoA to VideoB.
A conventional wipe generator has three main functional components, namely a function generator section 4, an arithmetic section 6 and a comparator section 10. The function generator section comprises a horizontal function generator 14H and a vertical function generator 14V, which generate predetermined functions synchronously with the horizontal and vertical sync signals respectively of the two synchronized input video signals. The functions that are generated by the function generators depend on the nature of the wipe that is desired.
The arithmetic section carries out mathematical operations on the functions provided by the function generators. The arithmetic section has a horizontal channel 16H and a vertical channel 16V. Each channel 16 includes an offset unit 18 for offsetting the function provided by the corresponding function generator 14, by adding a constant (negative, zero or positive) to the function, a scale unit 20 for scaling the function, by multiplying by a constant (greater than, less than, or equal to unity), an absolute value unit 22 for providing at the output the absolute value of the function received at the input, and a squaring unit 24. Since these functions of offsetting, scaling, forming the absolute value and forming the square are performed in the horizontal and vertical channels, they are performed separately on the output signals provided by the horizontal and vertical function generators 14H and 14V respectively.
The outputs of the scale unit 20H, absolute value unit 22H and square unit 24H are connected to a switch 28H for selecting the output of one of these units. Switch 28H is connected to a further switch 32H for applying the selected function signal to either a non-additive mixer 36 or a summation circuit 40. Switches 28V and 32V are similarly connected between, on the one hand, scale unit 20V, absolute value unit 22V and square unit 24V and, on the other hand, non-additive mixer 36 and summation circuit 40. A switch 46 is able to select the output of summation circuit 40, the output of mixer 36 or the output of a square root circuit 42, whose input is connected to the output of summation circuit 40. The function signal provided by switch 46 is the output signal of arithmetic section 6, and is known as a wipe solid.
The comparator section receives the wipe solid from the arithmetic section and also receives a signal whose value depends on the position of a lever arm 50 or other manual control member. These two input signals are applied to a comparator 54, in which a clip level is subtracted from the wipe solid level and the resulting difference signal is multiplied by a gain factor. The output signal of comparator 54 is level shifted at 56 and limited at 60 to provide a mix control signal which is applied to the control port of mixer 2.
As the lever arm is swung through its range of movement from one extreme to the other, the output signal VideoC of the mixer changes from VideoA to VideoB. When the lever arm is at an intermediate position, the VideoC field is divided spatially between VideoA and VideoB. The wipe solid determines the configuration of the transition in the VideoC field between VideoA and VideoB. For example, the wipe solid might be a cone, in which case the transition between VideoA and VideoB is a circle, and the size of the circle depends on the position of the lever arm.
The conventional wipe generator is subject to the disadvantage that the function generators are special purpose, and therefore multiple function generators are needed in order to achieve a wide variety of wipes. Further, the arithmetic unit must be able to perform a wide variety of mathematical operations on the functions. Although arithmetic section 6 shown in FIG. 1 includes eleven arithmetic units, there are numerous wipe solids that cannot be generated without additional circuitry.
Wipe solid generators of the general architecture shown in FIG. 1 have been implemented in both the analog domain and the digital domain.